1. Field of the Invention
The present invention relates to a solenoid-operated valve which has a movable iron core that is displaceable under a magnetically exciting action of a coil. In particular, the present invention relates to a solenoid-operated valve which makes it possible to improve the response performance when the movable iron core is displaced.
2. Description of the Related Art
A solenoid-operated valve has been hitherto adopted, in which a solenoid (electromagnet) is magnetically excited by applying a current, and a movable iron core is attracted and displaced toward a fixed iron core under the magnetically exciting action thereof so that the valve-open state and the valve-closed state of a valve plug are switched by the movable iron core.
Such a solenoid-operated valve is adopted, for example, for a classifier which selects and removes defectives, impurities and the like from rice grains or tealeaves, for example, which are in transport. In such a situation, the defectives or impurities, which are being transported on a transport passage, are detected by a detection sensor. A detection signal from the detection sensor is supplied to the solenoid-operated valve, and thus the solenoid-operated valve is in the valve-open state. Accordingly, the pressurized air is belched out from the solenoid-operated valve toward the transport passage, and the impurities or the like are removed by the pressurized air. A high speed response performance is required for the solenoid-operated valve so that the pressurized air can be belched out by performing the valve-opening/closing operation at a high speed in response to the detection signal from the detection sensor.
The solenoid-operated valve as described above comprises a solenoid section and a valve section. A plate-like movable iron core is provided displaceably in the axial direction in a valve chamber which is communicated with an input port and an output port. The solenoid section is provided above the movable iron core. The solenoid section includes a coil wound around a coil bobbin arranged substantially perpendicularly to the axis of the solenoid-operated valve. A fixed iron core is inserted and fixed to a central portion of the coil bobbin.
The movable iron core is pressed downwardly by the pressurized air supplied from the input port to give the valve-closed state in which the movable iron core is seated on a valve seat. When the magnetic field is generated between the movable iron core and the fixed iron core under the energization action exerted on the coil in the valve-closed state, the movable iron core is attracted toward the fixed iron core to give the valve-open state in which the movable iron core is separated from the valve seat (see, for example, Japanese Patent No. 2955520).
Generally, in the solenoid-operated valve as described above, there is a need for the displacement speed of the movable iron core in the axial direction, which is obtained when the valve plug is opened/closed, to be further increased in order to reliably remove the impurities or the like transported over the transport passage.
In the conventional technique concerning Japanese Patent No. 2955520, when the solenoid-operated valve is switched to the valve-closed state, then the energization of the coil is stopped, and the movable iron core is displaced in the direction directed to the valve seat so that the movable iron core is seated thereon by the air pressure of the pressurized air supplied from the input port to the valve chamber. In order to obtain the necessary high speed as described above, for example, the air pressure of the pressurized air supplied to the input port may be increased so that the displacement speed of the movable iron core is increased when the valve is closed.
However, the air pressure of the pressurized air essentially allows the pressurized air to flow from the input port to the output port in the valve-open state, and the pressurized air is discharged from the output port to remove the impurities or the like. Therefore, it is difficult to increase the air pressure so that the displacement speed of the movable iron core is increased when the valve is closed by the change of the air pressure, only for the purpose of increasing the displacement speed of the movable iron core.